refactor(deploy): rename vultr-deploy.yml → hetzner-deploy.yml + secrets

refactor(deploy): rename vultr-deploy.yml → hetzner-deploy.yml + secrets

Migration moved gluecron from a Vultr box (45.76.171.37) to a Hetzner
Cloud box (178.104.208.252). Workflow + setup script were still named
'vultr' which was actively misleading. Renamed everything to match
reality so future-you doesn't waste 10 minutes hunting.

Renamed:
- .github/workflows/vultr-deploy.yml → hetzner-deploy.yml
  - Workflow display name: 'Vultr Deploy' → 'Hetzner Deploy'
  - concurrency group: vultr-deploy → hetzner-deploy
  - Secret references: VULTR_HOST/USER/SSH_KEY → HETZNER_HOST/USER/SSH_KEY
  - Example IP in comments updated to 178.104.208.252

scripts/setup-auto-deploy.ps1
  - Comments + log messages: vultr → hetzner
  - 'gh secret set' calls now write to HETZNER_* names
  - Default $HOST_ updated to 178.104.208.252
  - Workflow trigger uses hetzner-deploy.yml

Tests: 1255 pass / 1 unrelated pre-existing fail. Workflow yaml is
syntactically valid.

NOTE: this commit assumes HETZNER_HOST/USER/SSH_KEY secrets exist in
GitHub. The rename in the workflow file means deploys WILL fail until
those secrets are populated. Owner runs:
  '178.104.208.252' | gh secret set HETZNER_HOST --repo ccantynz-alt/Gluecron.com
  'root'           | gh secret set HETZNER_USER --repo ccantynz-alt/Gluecron.com
  type $HOME\.ssh\gluecron_deploy_key | gh secret set HETZNER_SSH_KEY --repo ccantynz-alt/Gluecron.com